Densification of a powder-metal skeleton by transient liquid-phase infiltration

Transient liquid-phase infiltration (TLI) is a new method for densifying a powder-metal skeleton that produces a final part of homogeneous composition without significant dimensional change, offering advantages over traditional infiltration and full-density sintering. Fabrication of direct metal parts with complex geometry is possible using TLI in conjunction with solid freeform fabrication (SFF) processes such as three-dimensional printing, which produce net-shape powder-metal skeletons directly from computer-aided design models. The TLI method uses an infiltrant material similar in composition to the skeleton, but also containing a melting-point depressant (MPD), which allows the liquid metal to fill the skeleton void space and later facilitates homogenization. The materials requirements for such a system are discussed, and four experimental material systems were developed with final compositions of approximately Ni-40 wt pct Cu, Ni-4 wt pct Si, Fe-3 wt pct Si, and Fe-12 wt pct Cr-1 wt pct C, with copper, silicon, and carbon serving as the MPDs. Infiltration techniques include gating the introduction of liquid, saturating the melt to prevent erosion, and controlling variations in bulk composition along the infiltration path. Infiltration lengths exceeded 200 mm in the two nickel systems and exceeded 100 mm in the two iron systems. After infiltration, various heat treatments were conducted and mechanical properties were tested, including the tensile, hardness, and impact strength.     

Microstructural Evolution and Solidification Behavior of Al-Mg-Si Alloy in High-Pressure Die Casting

Microstructural evolution and solidification behavior of Al-5 wt pct Mg-1.5 wt pct Si-0.6 wt pct Mn-0.2 wt pct Ti alloy have been investigated using high-pressure die casting. Solidification commences with the formation of primary α-Al phase in the shot sleeve and is completed in the die cavity. The average size of dendrites and fragmented dendrites of the primary α-Al phase formed in the shot sleeve is 43 μm, and the globular primary α-Al grains formed inside the die cavity is at a size of 7.5 μm. Solidification inside the die cavity also forms the lamellar Al-Mg₂Si eutectic phase and the Fe-rich intermetallics. The size of the eutectic cells is about 10 μm, in which the lamellar α-Al phase is 0.41 μm thick. The Fe-rich intermetallic compound exhibits a compact morphology and is less than 2 μm with a composition of 1.62 at. pct Si, 3.94 at. pct Fe, and 2.31 at. pct Mn. A solute-enriched circular band is always observed parallel to the surface of the casting. The band zone separates the outer skin region from the central region of the casting. The solute concentration is consistent in the skin region and shows a general drop toward the center inside the band for Mg and Si. The peak of the solute enrichment in the band zone is much higher than the nominal composition of the alloy. The die casting exhibits a combination of brittle and ductile fracture. There is no significant difference on the fracture morphology in the three regions. The band zone is not significantly detrimental in terms of the fracture mechanism in the die casting. Calculations using the Mullins and Sekerka stability criterion reveal that the solidification of the primary α-Al phase inside the die cavity has been completed before the spherical α-Al globules begin to lose their stability, but the α-Al grains formed in the shot sleeve exceed the limit of spherical growth and therefore exhibit a dendritic morphology.     

Microstructural refinement of an As-cast Al-12.6 wt pct Si alloy by repeated thermomechanical treatment to produce a heavily deformable material

A certain degree of cold working is advantageous in developing a fine microstructure with minute silicon crystals for eutectic and/or hypereutectic Al-Si cast alloys. A novel process, repeated thermomechanical treatment (RTMT), was applied to an Al-12.6 wt pct Si cast alloy. The process involves multiple-pass cold working (less than a 20 pct reduction in section area) and heat treatment at 793 K for 3.6 ks. Cold-work annealing was repeated up to about an 80 pct reduction in section from the beginning. The RTMT material showed a refined microstructure with high ductility. Most silicon crystals were fragmented to only a few micrometers and were spheroidized. The RTMT material showed such marked plasticity that it could be wrought up to a 99 pct reduction in section at room temperature. The Cold-worked RTMT materials exhibited an excellent balance between tensile strength and elongation and a higher strain hardening than the cast material.     

Crystallization studies of the β (Mg2Pb) phase and its phase boundaries in the Pb-Mg-Bi system

A peritectic reaction between Mg bismuthide and Mg plumbide changes from an even to an odd reaction at 310 °C (0.14 wt pct Bi and 4 wt pct Mg) owing to the incorporation of 1.6 wt pct Bi into Mg plumbide; this value was calculated from material balances of equilibrium studies of this reaction and confirmed by direct analysis of crystals using a “Cameca Microbeam” electroprobe microanalyzer. A model is presented in which an atom of Pb in the unit cell, 46 Mg₂Pb Mg₂Pb₅, is replaced by an atom of Bi which gives a concentration of 1.63 to 1.71 wt pct Bi depending on the actual species of Mg plumbide present. The phase boundary for double saturation with Pb and Mg plumbide, established from the data of equilibrium tests, shows a minimum temperature of 251.8 °C at 0.008 wt pct Bi and 2.2 wt pct Mg. Alloys in the primary Pb phase field adjacent to this boundary show undercooling to less than 248.5 °C followed by one or two sharp temperature increases to 250.5 °C, with the initiation of the removal of Bi when double saturation occurs giving a final liquid phase containing less than 0.001 wt pct Bi. Crystallization paths for alloys in the Mg plumbide phase field show a catatectic reaction and polymorphic transformations in the intermetallic compound. The removal of Bi is dependent on the concentration of Bi and Mg in the initial alloy. In systems containing sufficient Mg, a final alloy containing less than 0.001 wt pct Bi can be produced, and these conditions have been used as the starting point for the development of a process for the removal of Bi from Pb. Finally, the crystallization paths show there is a change in the thermal properties of the liquid alloy at 0.008 wt pct Bi which is independent of the temperature and concentration of Mg, and further work is required to resolve this finding.     

Determination of the melting and solidification characteristics of solders using differential scanning calorimetry

Differential scanning calorimetry (DSC) is used in the present study to determine the onset temperature of phase transformation and the enthalpy of fusion of various solder alloys. The solders studied are Sn-Pb, Sn-Bi, Ag-Sn, In-Ag, and Sn-Pb-Bi alloys. Very notable undercooling, such as 35 °C, is observed in the solidification process; however, a superheating effect is not as significant in the heating process. Besides the direct measurements of reaction temperature and heat of fusion, the fraction solid vs temperature has also been determined using a DSC coupled with a mathematical-model method. The heating and cooling curves of the solders are first determined using DSC. By mathematically modeling the heat transfer of the DSC cells, the heat evolution and absorption can be calculated, and then the melting and solidification curves of the solder alloys are determined. The three ternary alloys, Sn-35 wt pct Pb-10 wt pct Bi, Sn-45 wt pct Pb-10 wt pct Bi, and Sn-55 wt pct Pb-10 wt pct Bi, displayed similar DSC cooling curves, which had three reaction peaks. However, the solid fractions of the three alloys at the same temperature in the semisolid state, which had been determined quantitatively using the DSC coupled with a mathematical method, were different, and their primary solidification phases were also different.     

The effect of cold work on the precipitation of Ω and θ′ in a ternary Al-Cu-Mg alloy

The effect of plastic deformation on the microstructural evolution of an Al-5.0Cu-0.5Mg (wt pct) ternary alloy was investigated. Hardness measurements and quantitative precipitate analysis were performed on specimens that were water quenched from a solution heat treatment, stretched either 0 or 6 pct and immediately aged at ambient temperature or artificially aged at 200 °C or 250 °C for times up to 3000 hours. Quantitative transmission electron microscopy (TEM) was used to characterize Ω and θ′ number density, diameter, and thickness as a function of preage mechanical stretch and artificial aging condition. Age hardening curves for naturally and artificially aged specimens revealed an increase in hardness corresponding with a preage stretch. Quantitative TEM verified an increase in the number density and a refinement of precipitates for both Ω and θ′ between the 0 and 6 pct stretch condition for those samples artificially aged. When aged at 200 °C, θ′ exhibited superior coarsening resistance relative to the Ω phase. The quantified Ω coarsening kinetics were greater than similar Ag-containing alloys. To investigate the effects of trace Si additions on subsequent microstructural evolution, a series of Al-Cu-Mg-Si quaternary alloys were produced. The addition of 0.1Si (wt pct) was found to suppress Ω precipitation in most Al-4.0Cu-xMg alloys investigated. These initial results indicate that Ω precipitation may be related to the Mg/Si ratio.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Metallurgical and Materials Transactions A - Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature...     

Influence of Microstructural Length Scale on the Strength and Annealing Behavior of Pearlite, Bainite, and Martensite

A medium carbon steel containing 0.4 pct C, 2.9 pct Mn, and 1.9 pct Si (wt. pct) was heat treated to produce pearlitic, bainitic, and martensitic microstructures. The three microstructures were cold rolled to large strains in order to investigate the change in strength/hardness as a result of the refinement of the microstructural length scale due to plastic deformation. The results show that the hardness does not saturate in any of the above microstructures, implying that geometric strengthening plays an important role not only in lamellar microstructures such as pearlite, but also in lath-type microstructures such as bainite and martensite. Low temperature annealing of the deformed microstructures revealed that they are very resistant to recrystallization.     

Influence of chromium and impurities on the grain- refining behavior of aluminum

The grain-refining behavior of high purity aluminum (HPA1) and commercial purity aluminum (CPA1) containing Fe and Si as impurities (<0.2 wt pct each) has been studied with and without the presence of Cr in small and large quantities (0.2 and 2 wt pct). The Al-5Ti-lB master alloy ingot (0.2 wt pct) was used as a grain refiner. The emphasis was on the influence of individual elements and their interactions with the other elements on the grain-refining behavior of Al. Good grain refinement with insignificant fading in CPA1 was observed in comparison to HPA1. Similar results were obtained with a small concentration of Cr in HPA1 in HPA1-0.2 wt pct Cr alloy. The CPA1 and HPA1-0.2 wt pct Cr alloy have given the best grain-refining results among all the cases studied. A combination of small quantities of Fe, Si, and Cr (CPA1-0.2 wt pct Cr) has shown early and significant fading. A large concentration of Cr (2 wt pct) has shown a poisoning effect irrespective of the presence or absence of impurities such as Fe and Si in Al. Thus, Cr was found to be beneficial for grain refinement at smaller concentrations in the absence of impurities. But at higher concentrations of Cr, it had an adverse effect,i.e., led to coarser grain sizes both in the presence and absence of impurities.     

A 3rd Generation Advanced High-Strength Steel (AHSS) Produced by Dual Stabilization Heat Treatment (DSHT)

A 3rd generation advanced high-strength steel containing, in wt pct, 0.3 C, 4.0 Mn, 1.5 Al, 2.1 Si, and 0.5 Cr has been produced using a dual stabilization heat treatment—a five stage thermal processing schedule compatible with continuous galvanized steel production. In excess of 30 vol pct retained austenite containing at least 0.80 wt pct C was achieved with this alloy, which had tensile strengths up to 1650 MPa and tensile elongations around 20 pct.     

Heat flux transients at the casting/chill interface during solidification of aluminum base alloys

Heat flow at the metal/chill interface of bar-type castings of aluminum base alloys was modeled as a function of thermophysical properties of the chill material and its thickness. Experimental setup for casting square bars of Al-13.2 pct Si eutectic and Al-3 pet Cu-4.5 pct Si long freezing range alloys with chill at one end exposed to ambient conditions was fabricated. Experiments were carried out for different metal/chill combinations with and without coatings. The thermal history at nodal locations in the chill obtained during the experiments was used to estimate the interface heat flux by solving a one-dimensional Fourier heat conduction equation inversely. Using the data on transient heat flux q, the heat flow at the casting/chill interface was modeled in two steps: (1) The peak in the heat flux curve q_max was modeled as a power function of the ratio of the chill thickness d to its thermal diffusivity a, and (2) the factor (q/q_max) X α^0.05 was also modeled as a power function of the time after the solidification set in. The model was validated for Cu-10 pct Sn -2 pct Zn alloy chill and Al-13.2 pct Si and Al-3 pct Cu-4.5 pct Si as the casting alloys. The heat flux values estimated using the model were used as one of the boundary conditions for solidification simulation of the test casting. The experimental and simulated temperature distributions inside the casting were found to be in good agreement. Formerly Assistant Professor with Karnataka Regional Engineering College     

Fracture resistance of low-carbon alloy irons

The temperature dependence of the critical stress intensity factor and of the fracture energy were measured on six low-carbon iron alloys, one containing 0.002 wt pct C and five containing 0.02 wt pct C. Either Ni, P, Si, or Si and Mn were added to four of the five 0.02C irons in quantities typically found in ferritic steels. The fracture tests were conducted at rapid (but less than impact) speed of 1 ips on fatigue cracked, three-point bend beam specimens. Each alloy was tested over a temperature range of —195° to 24°C in both furnace-cooled and quench-aged states. Both alloying and heat treatment produced wide differences in the fracture resistance of these alloys. The quench-aged 0.002C iron and furnace-cooled phosphorus alloy failed by intergranular separation, whereas the remaining alloys exhibited cleavage fractures. With the exception of 0.002C iron, an alloy in the quench-aged condition had higher fracture toughness than the same alloy in the furnace-cooled state. The transition temperature, however, was influenced by heat treatment only in the plain carbon irons. In this case the transition temperature was independent of carbon content but the furnace-cooled specimen had a lower transition temperature than the quench-aged specimens. D. C. A. R. COX, formerly Exchange Scientist at the Naval Research Laboratory     

Thermal transformations in mechanically alloyed Fe-Zn-Si materials

The ball milling of elemental powders corresponding to Γ (Fe₃Zn₁₀)+0.12 wt pct Si; Γ₁ (Fe₅Zn₂₁) + 0.12 wt pct Si; δ (FeZn₇)+0.12 wt pct Si; and ζ (FeZn₁₃)+0.12 wt pct Si composition ratios yields crystalline, mechanically alloyed phases. Differential scanning calorimetry (DSC) measurements of these materials show that they evolve differently, with well-defined characteristic stages. The activation energies for processes corresponding to these stages, based on kinetic analyses, are determined and correlated to microstructural evolvements. The processes occurring during the first stage below 250 °C, for all of the materials studied using X-ray diffraction (XRD) analysis, are associated with release of strain, recovery, and limited atomic diffusion. The activation energies for recovery processes are 120 kJ/mole for the Γ+0.12 wt pct Si, 131 kJ/mole for δ+0.12 wt pct Si, and 96 kJ/mole for ζ+0.12 wt pct Si alloys. At higher temperatures, recrystallization and other structural transformations occur with activation energies of 130 and 278 kJ/mole for Γ+0.12 wt % Si; of 161 kJ/mole for Γ₁+0.12 wt pct Si; of 167 and 244 kJ/mole for δ+0.12 wt pct Si; and of 641 kJ/mole for the ζ+0.12 wt pct Si. In addition, a eutectic reaction at 420 °C±3 °C, corresponding to the Zn-Si system, and a melting of Zn in Fe-Zn systems are observed for the ζ+0.12 wt pct Si material. The relation of FeSi formation in the Sandelin process is discussed.     

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al. Formerly with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University     

The atomic-structure changes in Al-16 pct Si alloy above the liquidus

The atomic-structure changes in an Al-16 pct Si alloy above the liquidus have been studied by a θ-θ high-temperature X-ray diffractometer, rapid solidification, a vertical centrifugal casting apparatus, and differential scanning calorimetry (DSC) measurements. It was found that the diffraction intensity and structure-factor (S(Q)) curves for an Al-16 pct Si alloy have small prepeaks at small Q values when the temperature is high enough. Rapid-solidification and centrifugal-casting experimental results show that the primary silicon phase can easily coarsen and segregate under additive force after an overheat at high temperatures. The DSC measurements show that the temperature and latent heat of primary solidification rise with the temperature of overheating. These experimental results suggest that a strong interaction occurs between Si-Si atoms in a liquid Al-16 pct Si alloy at high temperatures, resulting in the microsegregation of Si atoms in the melt.     

Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy

The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of 550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries. However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite. The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S₂ by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.     

Improved CFD Model to Predict Flow and Temperature Distributions in a Blast Furnace Hearth

The campaign life of a blast furnace is limited by the erosion of hearth refractories. Flow and temperature distributions of the liquid iron have a significant influence on the erosion mechanism. In this work, an improved three-dimensional computational fluid dynamics model is developed to simulate the flow and heat transfer phenomena in the hearth of BlueScope’s Port Kembla No. 5 Blast Furnace. Model improvements feature more justified input parameters in turbulence modeling, buoyancy modeling, wall boundary conditions, material properties, and modeling of the solidification of iron. The model is validated by comparing the calculated temperatures with the thermocouple data available, where agreements are established within ±3 pct. The flow distribution in the hearth is discussed for intact and eroded hearth profiles, for sitting and floating coke bed states. It is shown that natural convection affects the flow in several ways: for example, the formation of (a) stagnant zones preventing hearth bottom from eroding or (b) the downward jetting of molten liquid promoting side wall erosion, or (c) at times, a vortex-like peripheral flow, promoting the “elephant foot” type erosion. A significant influence of coke bed permeability on the macroscopic flow pattern and the refractory temperature is observed.